The invention relates to injection-molding machines for injecting synthetic plastic material in plastic condition into a mold cavity, particularly using a plastifying and injecting screw driven by an hydraulic cylinder-and-piston unit. Still more particularly, the invention relates to the problem of regulating the pressure in the hydraulic cylinder in dependence upon a signal furnished by a desired-value signal generator.
Already known is an injection-molding machine provided with a programming arrangement. Under the control of the programming arrangement, an hydraulically activatable screw performs first the mold-filling phase during which resort is had to a distance-dependent velocity regulation (negative-feedback control of velocity). The mold-filling phase is followed by the pressing phase during which the hydraulic cylinder pressure is regulated in dependence upon pressure. With this known arrangement, only the pressure in the hydraulic cylinder is taken into account, even though this pressure will during certain portions of the injection-molding cycle deviate very markedly from the pressure in the mold cavity. This discrepancy between the pressures in the drive cylinder and in the mold cavity leads frequently to inferior quality of the molded finished product and/or to reproducibility difficulties. The difference which develops during the molding cycle as between the pressures in the hydraulic drive cylinder, on the other hand, and in the mold cavity, on the other hand, is attributable mainly to the viscosity changes undergone by the material in the mold cavity as such material is subjected to temperature changes, and also to differences in the charges employed.
It is also known to control an injection-molding machine by measuring the pressure in the mold cavity and in dependence thereon controlling the fluid pressure in the hydraulic cylinder. However, the heretofore utilized pressure control expedients of this type did not involve a complete closed-loop regulation (negative-feedback control). Also, the pressure control worked only during the phase in which synthetic plastic material is filled into and pressed solid in the mold cavity, with a predetermined pressure point in the pressure rise phase being compared with a desired value of pressure, so that in the case of an error signal the volumetric throughput of material to the mold cavity can be appropriately altered while the injection phase is still actually in progress. This known control technique rests on the assumption that upon reaching of a predetermined pressure value in an initial portion of the cycle it will automatically happen that in the subsequent course of the cycle a second predetermined command value will actually be reached. A disadvantage of this approach is that despite the efforts to the contrary there can develop within the mold cavity pressures having a detrimental influence upon the actual course of performance of the cycle itself or upon the quality of the finished product.
It is also known to provide an injection-molding machine with a closed-loop regulating circuit (negative-feedback control circuit) for continuously regulating (by means of negative feedback) the pressure in the mold cavity. However, this particular prior-art arrangement has the disadvantage that subsequent to the mold-filling phase, as the material in the mold begins to set during the compression phase, it becomes impossible to perform pressure measurements of absolute reliability. This is particularly the case when the material undergoing setting shrinks and moves out of contact with a pressure-measuring device located within the mold cavity or flush with the wall of the mold cavity; as the shrinking material moves out of contact with the pressure-measuring device, the latter generates a signal indicative of very low or even zero pressure, which by virtue of the negative-feedback control causes the hydraulic cylinder pressure to be increased to its maximum value. This can lead to damage of the molded mass in the mold cavity.
A somewhat similar problem is presented by the effect of the mold clamping force upon the performance of such pressure measurements. As the mass of material in the mold sets, it may expand. Such expansion is resisted by the mold clamping force and can lead to the development of pressure-indicating signals greatly exceeding the corresponding desired-value signals. The resultant error signals then cause the hydraulic drive for the injection screw to retract somewhat. However, retraction of the screw drive under such circumstances does not result in a corresponding decrease in the measured mold cavity pressure, and so the screw drive will not cease retracting until it reaches its most retracted end position. However, such premature retraction disturbs the molding cycle and may have a detrimental effect upon the setting process occurring within the mold cavity.